Are you talking about painting a strip a couple of inches above the current top of your bottom coat?
I have heard of painting the bottom coat a little higher to reflect wave action etc and keep growth off the gelcoat of the sides, but painting a stripe a few inches above the current waterline will still leave those few inches of exposed gelcoat.
I guess another possibility, which I have seen on several occasions, and used once on a boat I race on, is to stop the bottom coat at the current location, paint white bottom coat pain (such as Vivid) just above this for another inch or two. Then you could paint a stripe above this. Maybe that would help with what you are asking about.
Saginaw Bay, Michigan
"The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails." - William Arthur Ward